postgresql/doc/src/sgml/xplang.sgml

 <chapter id="xplang">
  <title id="xplang-title">Procedural Languages</title>

<!-- **********
     * General information about procedural language support
     **********
-->

<para>
    Beginning with the release of version 6.3,
    <productname>Postgres</productname> supports
    the definition of procedural languages.
    In the case of a function or trigger
    procedure defined in a procedural language, the database has
    no builtin knowlege how to interpret the functions source
    text. Instead, the calls are passed into
    a handler that knows the details of the language. The
    handler itself is a special programming language function
    compiled into a shared object
    and loaded on demand.
</para>

<!-- **********
     * Installation of procedural languages
     **********
-->

<sect1>
<title>Installing Procedural Languages</title>

<procedure>
    <title>
    Procedural Language Installation
    </title>

    <para>
    A procedural language is installed in the database in three steps.
    </para>
	<step performance="Required">
	    <para>
	        The shared object for the language handler
		must be compiled and installed. By default the
		handler for PL/pgSQL is built and installed into the
		database library directory. If Tcl/Tk support is
		configured in, the handler for PL/Tcl is also built
		and installed in the same location.
	    </para>
	    <para>
	        Writing a handler for a new procedural language (PL)
		is outside the scope of this manual. 
	    </para>
	</step>
	<step performance="Required">
	    <para>
	        The handler must be declared with the command
		<programlisting>
    CREATE FUNCTION <replaceable>handler_function_name</replaceable> () RETURNS OPAQUE AS
        '<filename>path-to-shared-object</filename>' LANGUAGE 'C';
		</programlisting>
		The special return type of <acronym>OPAQUE</acronym> tells
		the database, that this function does not return one of
		the defined base- or composite types and is not directly usable
		in <acronym>SQL</acronym> statements.
	    </para>
	</step>
	<step performance="Required">
	    <para>
	    	The PL must be declared with the command
		<programlisting>
    CREATE [ TRUSTED ] PROCEDURAL LANGUAGE '<replaceable>language-name</replaceable>'
        HANDLER <replaceable>handler_function_name</replaceable>
	LANCOMPILER '<replaceable>description</replaceable>';
		</programlisting>
		The optional keyword <acronym>TRUSTED</acronym> tells
		if ordinary database users that have no superuser
		privileges can use this language to create functions
		and trigger procedures. Since PL functions are
		executed inside the database backend it should only be used for
		languages that don't gain access to database backends
		internals or the filesystem. The languages PL/pgSQL and
		PL/Tcl are known to be trusted.
	    </para>
	</step>
</procedure>

<procedure>
    <title>Example</title>
    <step performance="Required">
    <para>
        The following command tells the database where to find the 
	shared object for the PL/pgSQL languages call handler function.
    </para>
    <programlisting>
    CREATE FUNCTION plpgsql_call_handler () RETURNS OPAQUE AS
        '/usr/local/pgsql/lib/plpgsql.so' LANGUAGE 'C';
    </programlisting>
    </step>

    <step performance="Required">
    <para>
        The command
    </para>
    <programlisting>
    CREATE TRUSTED PROCEDURAL LANGUAGE 'plpgsql'
        HANDLER plpgsql_call_handler
        LANCOMPILER 'PL/pgSQL';
    </programlisting>
    <para>
        then defines that the previously declared call handler
	function should be invoked for functions and trigger procedures
	where the language attribute is 'plpgsql'.
    </para>
    <para>
        PL handler functions have a special call interface that is
	different from regular C language functions. One of the arguments
	given to the handler is the object ID in the <filename>pg_proc</filename>
	tables entry for the function that should be executed.
        The handler examines various system catalogs to analyze the
	functions call arguments and it's return data type. The source
	text of the functions body is found in the prosrc attribute of
	<filename>pg_proc</filename>.
	Due to this, in contrast to C language functions, PL functions
	can be overloaded like SQL language functions. There can be
	multiple different PL functions having the same function name,
	as long as the call arguments differ.
    </para>
    <para>
        Procedural languages defined in the <filename>template1</filename>
	database are automatically defined in all subsequently created
	databases. So the database administrator can decide which
	languages are available by default.
    </para>
    </step>
</procedure>
</sect1> <!-- **** End of PL installation **** -->

<!-- **********
     * The procedural language PL/pgSQL
     **********
-->

<sect1>
<title>PL/pgSQL</title>

<para>
    PL/pgSQL is a loadable procedural language for the
    <productname>Postgres</productname> database system.
</para>

<para>
    This package was originally written by Jan Wieck.
</para>

<!-- **** PL/pgSQL overview **** -->

<sect2>
<title>Overview</title>

<para>
    The design goals of PL/pgSQL were to create a loadable procedural
    language that
    <itemizedlist>
    <listitem>
	<para>
        can be used to create functions and trigger procedures,
	</para>
    </listitem>
    <listitem>
	<para>
        adds control structures to the <acronym>SQL</acronym> language,
	</para>
    </listitem>
    <listitem>
	<para>
        can perform complex computations,
	</para>
    </listitem>
    <listitem>
	<para>
        inherits all user defined types, functions and operators,
	</para>
    </listitem>
    <listitem>
	<para>
        can be defined to be trusted by the server,
	</para>
    </listitem>
    <listitem>
	<para>
        is easy to use.
	</para>
    </listitem>
    </itemizedlist>
</para>
<para>
    The PL/pgSQL call handler parses the functions source text and
    produces an internal binary instruction tree on the first time, the
    function is called by a backend. The produced bytecode is identified
    in the call handler by the object ID of the function. This ensures,
    that changing a function by a DROP/CREATE sequence will take effect
    without establishing a new database connection. 
</para>
<para>
    For all expressions and <acronym>SQL</acronym> statements used in
    the function, the PL/pgSQL bytecode interpreter creates a
    prepared execution plan using the SPI managers SPI_prepare() and
    SPI_saveplan() functions. This is done the first time, the individual
    statement is processed in the PL/pgSQL function. Thus, a function with
    conditional code that contains many statements for which execution
    plans would be required, will only prepare and save those plans
    that are really used during the entire lifetime of the database
    connection.
</para>
<para>
    Except for input-/output-conversion and calculation functions
    for user defined types, anything that can be defined in C language
    functions can also be done with PL/pgSQL. It is possible to
    create complex conditional computation functions and later use
    them to define operators or use them in functional indices.
</para>
</sect2>

<!-- **** PL/pgSQL Description **** -->

<sect2>
<title>Description</title>

<!-- **** PL/pgSQL structure **** -->

<sect3>
<title>Structure of PL/pgSQL</title>

<para>
    The PL/pgSQL language is case insensitive. All keywords and
    identifiers can be used in mixed upper- and lowercase.
</para>
<para>
    PL/pgSQL is a block oriented language. A block is defined as

<programlisting>
    [&lt;&lt;label&gt;&gt;]
    [DECLARE
        <replaceable>declarations</replaceable>]
    BEGIN
        <replaceable>statements</replaceable>
    END;
</programlisting>

    There can be any number of subblocks in the statement section
    of a block. Subblocks can be used to hide variables from outside a
    block of statements. The variables
    declared in the declarations section preceding a block are
    initialized to their default values every time the block is entered,
    not only once per function call.
</para>
  
<para>
    It is important not to misunderstand the meaning of BEGIN/END for
    grouping statements in PL/pgSQL and the database commands for
    transaction control. Functions and trigger procedures cannot
    start or commit transactions and <productname>Postgres</productname>
    does not have nested transactions.
</para>
</sect3>

<!-- **** PL/pgSQL comments **** -->

<sect3>
<title>Comments</title>

<para>
    There are two types of comments in PL/pgSQL. A double dash '--'
    starts a comment that extends to the end of the line. A '/*'
    starts a block comment that extends to the next occurence of '*/'.
    Block comments cannot be nested, but double dash comments can be
    enclosed into a block comment and a double dash can hide
    the block comment delimiters '/*' and '*/'.
</para>
</sect3>

<!-- **** PL/pgSQL declarations **** -->

<sect3>
<title>Declarations</title>

<para>
    All variables, rows and records used in a block or it's
    subblocks must be declared in the declarations section of a block
    except for the loop variable of a FOR loop iterating over a range
    of integer values. Parameters given to a PL/pgSQL function are
    automatically declared with the usual identifiers $n.
    The declarations have the following syntax:
</para>
<variablelist>

<varlistentry>
<term>
<replaceable>name</replaceable> [ CONSTANT ]
<replaceable>>typ</replaceable>> [ NOT NULL ] [ DEFAULT | :=
	<replaceable>value</replaceable> ];
</term>
<listitem>
<para>
    Declares a variable of the specified base type. If the variable
    is declared as CONSTANT, the value cannot be changed. If NOT NULL
    is specified, an assignment of a NULL value results in a runtime
    error. Since the default value of all variables is the
    <acronym>SQL</acronym> NULL value, all variables declared as NOT NULL
    must also have a default value specified.
</para>
<para>
    The default value is evaluated ever time the function is called. So
    assigning '<replaceable>now</replaceable>' to a variable of type
    <replaceable>datetime</replaceable> causes the variable to have the
    time of the actual function call, not when the function was
    precompiled into it's bytecode.
</para>
</listitem>
</varlistentry>

<varlistentry>
<term>
<replaceable>name</replaceable> <replaceable>class</replaceable>%ROWTYPE;
</term>
<listitem>
<para>
    Declares a row with the structure of the given class. Class must be
    an existing table- or viewname of the database. The fields of the row
    are accessed in the dot notation. Parameters to a function can
    be composite types (complete table rows). In that case, the
    corresponding identifier $n will be a rowtype, but it
    must be aliased using the ALIAS command described below. Only the user
    attributes of a table row are accessible in the row, no Oid or other
    system attributes (hence the row could be from a view and view rows
    don't have useful system attributes).
</para>
<para>
    The fields of the rowtype inherit the tables fieldsizes 
    or precision for char() etc. data types.
</para>
</listitem>
</varlistentry>

<varlistentry>
<term>
<replaceable>name</replaceable> RECORD;
</term>
<listitem>
<para>
    Records are similar to rowtypes, but they have no predefined structure.
    They are used in selections and FOR loops to hold one actual
    database row from a SELECT operation. One and the same record can be
    used in different selections. Accessing a record or an attempt to assign
    a value to a record field when there is no actual row in it results
    in a runtime error.
</para>
<para>
    The NEW and OLD rows in a trigger are given to the procedure as
    records. This is necessary because in <productname>Postgres</productname>
    one and the same trigger procedure can handle trigger events for
    different tables.
</para>
</listitem>
</varlistentry>

<varlistentry>
<term>
<replaceable>name</replaceable> ALIAS FOR $n;
</term>
<listitem>
<para>
    For better readability of the code it is possible to define an alias
    for a positional parameter to a function.
</para>
<para>
    This aliasing is required for composite types given as arguments to
    a function. The dot notation $1.salary as in SQL functions is not
    allowed in PL/pgSQL.
</para>
</listitem>
</varlistentry>

<varlistentry>
<term>
RENAME <replaceable>oldname</replaceable> TO <replaceable>newname</replaceable>;
</term>
<listitem>
<para>
    Change the name of a variable, record or row. This is useful
    if NEW or OLD should be referenced by another name inside a 
    trigger procedure.
</para>
</listitem>
</varlistentry>

</variablelist>
</sect3>

<!-- **** PL/pgSQL data types **** -->

<sect3>
<title>Data Types</title>

<para>
    The type of a varible can be any of the existing basetypes of
    the database. <replaceable>type</replaceable> in the declarations
    section above is defined as:
</para>
<para>
    <itemizedlist>
    <listitem>
        <para>
	<productname>Postgres</productname>-basetype
	</para>
    </listitem>
    <listitem>
        <para>
	<replaceable>variable</replaceable>%TYPE
	</para>
    </listitem>
    <listitem>
        <para>
	<replaceable>class.field</replaceable>%TYPE
	</para>
    </listitem>
    </itemizedlist>
</para>
<para>
    <replaceable>variable</replaceable> is the name of a variable,
previously declared in the 
    same function, that is visible at this point.
</para>
<para>
    <replaceable>class</replaceable> is the name of an existing table
    or view where <replaceable>field</replaceable> is the name of
    an attribute.
</para>
<para>
    Using the <replaceable>class.field</replaceable>%TYPE
    causes PL/pgSQL to lookup the attributes definitions at the
    first call to the funciton during the lifetime of a backend.
    Have a table with a char(20) attribute and some PL/pgSQL functions
    that deal with it's content in local variables. Now someone
    decides that char(20) isn't enough, dumps the table, drops it,
    recreates it now with the attribute in question defined as
    char(40) and restores the data. Ha - he forgot about the
    funcitons. The computations inside them will truncate the values
    to 20 characters. But if they are defined using the
    <replaceable>class.field</replaceable>%TYPE
    declarations, they will automagically handle the size change or
    if the new table schema defines the attribute as text type.
</para>
</sect3>

<!-- **** PL/pgSQL expressions **** -->

<sect3>
<title>Expressions</title>

<para>
    All expressions used in PL/pgSQL statements are processed using
    the backends executor. Expressions which appear to contain
constants may in fact require run-time evaluation (e.g. 'now' for the
datetime type) so
it is impossible for the PL/pgSQL parser
    to identify real constant values other than the NULL keyword. All
    expressions are evaluated internally by executing a query
    <programlisting>
    SELECT <replaceable>expression</replaceable>
    </programlisting>
    using the SPI manager. In the expression, occurences of variable
    identifiers are substituted by parameters and the actual values from
    the variables are passed to the executor in the parameter array. All
    expressions used in a PL/pgSQL function are only prepared and
    saved once.
</para>
<para>
    The type checking done by the <productname>Postgres</productname>
    main parser has some side
    effects to the interpretation of constant values. In detail there
    is a difference between what the two functions

    <programlisting>
    CREATE FUNCTION logfunc1 (text) RETURNS datetime AS '
        DECLARE
            logtxt ALIAS FOR $1;
        BEGIN
            INSERT INTO logtable VALUES (logtxt, ''now'');
            RETURN ''now'';
        END;
    ' LANGUAGE 'plpgsql';
    </programlisting>

    and

    <programlisting>
    CREATE FUNCTION logfunc2 (text) RETURNS datetime AS '
        DECLARE
            logtxt ALIAS FOR $1;
            curtime datetime;
        BEGIN
            curtime := ''now'';
            INSERT INTO logtable VALUES (logtxt, curtime);
            RETURN curtime;
        END;
    ' LANGUAGE 'plpgsql';
    </programlisting>

    do. In the case of logfunc1(), the <productname>Postgres</productname>
    main parser
    knows when preparing the plan for the INSERT, that the string 'now'
    should be interpreted as datetime because the target field of logtable
    is of that type. Thus, it will make a constant from it at this time
    and this constant value is then used in all invocations of logfunc1()
    during the lifetime of the backend. Needless to say that this isn't what the
    programmer wanted.
</para>
<para>
    In the case of logfunc2(), the <productname>Postgres</productname> 
    main parser does not know
    what type 'now' should become and therefor it returns a datatype of
    text containing the string 'now'. During the assignment
    to the local variable curtime, the PL/pgSQL interpreter casts this
    string to the datetime type by calling the text_out() and datetime_in()
    functions for the conversion.
</para>
<para>
    This type checking done by the <productname>Postgres</productname> main
    parser got implemented after PL/pgSQL was nearly done.
    It is a difference between 6.3 and 6.4 and affects all functions
    using the prepared plan feature of the SPI manager.
    Using a local
    variable in the above manner is currently the only way in PL/pgSQL to get
    those values interpreted correctly.
</para>
<para>
    If record fields are used in expressions or statements, the data types of
    fields should not change between calls of one and the same expression.
    Keep this in mind when writing trigger procedures that handle events
    for more than one table.
</para>
</sect3>

<!-- **** PL/pgSQL statements **** -->

<sect3>
<title>Statements</title>

<para>
    Anything not understood by the PL/pgSQL parser as specified below
    will be put into a query and sent down to the database engine
    to execute. The resulting query should not return any data.
</para>

<variablelist>

<varlistentry>
<term>
Assignment
</term>
<listitem>
<para>
    An assignment of a value to a variable or row/record field is
    written as
    <programlisting>
    <replaceable>identifier</replaceable> := <replaceable>expression</replaceable>;
    </programlisting>
    If the expressions result data type doesn't match the variables
    data type, or the variable has a size/precision that is known
    (as for char(20)), the result value will be implicitly casted by
    the PL/pgSQL bytecode interpreter using the result types output- and
    the variables type input-functions. Note that this could potentially
    result in runtime errors generated by the types input functions.
</para>
<para>
    An assignment of a complete selection into a record or row can
    be done by
    <programlisting>
    SELECT <replaceable>expressions</replaceable> INTO <replaceable>target</replaceable> FROM ...;
    </programlisting>
    <replaceable>target</replaceable> can be a record, a row variable or a
    comma separated list of variables and record-/row-fields.
</para>
<para>
    if a row or a variable list is used as target, the selected values
    must exactly match the structure of the target(s) or a runtime error
    occurs. The FROM keyword can be followed by any valid qualification,
    grouping, sorting etc. that can be given for a SELECT statement.
</para>
<para>
    There is a special variable named FOUND of type bool that can be used
    immediately after a SELECT INTO to check if an assignment had success.

    <programlisting>
    SELECT * INTO myrec FROM EMP WHERE empname = myname;
    IF NOT FOUND THEN
        RAISE EXCEPTION ''employee % not found'', myname;
    END IF;
    </programlisting>

    If the selection returns multiple rows, only the first is moved
    into the target fields. All others are silently discarded.
</para>
</listitem>
</varlistentry>

<varlistentry>
<term>
Calling another function
</term>
<listitem>
<para>
    All functions defined in a <productname>Prostgres</productname>
    database return a value. Thus, the normal way to call a function
    is to execute a SELECT query or doing an assignment (resulting
    in a PL/pgSQL internal SELECT). But there are cases where someone
    isn't interested int the functions result.
    <programlisting>
    PERFORM <replaceable>query</replaceable>
    </programlisting>
    executes a 'SELECT <replaceable>query</replaceable>' over the
    SPI manager and discards the result. Identifiers like local
    variables are still substituted into parameters.
</para>
</listitem>
</varlistentry>

<varlistentry>
<term>
Returning from the function
</term>
<listitem>
<para>
    <programlisting>
    RETURN <replaceable>expression</replaceable>
    </programlisting>
    The function terminates and the value of <replaceable>expression</replaceable>
    will be returned to the upper executor. The return value of a function
    cannot be undefined. If control reaches the end of the toplevel block
    of the function without hitting a RETURN statement, a runtime error
    will occur.
</para>
<para>
    The expressions result will be automatically casted into the
    functions return type as described for assignments.
</para>
</listitem>
</varlistentry>

<varlistentry>
<term>
Aborting and messages
</term>
<listitem>
<para>
    As indicated in the above examples there is a RAISE statement that
    can throw messages into the <productname>Postgres</productname>
    elog mechanism.
    <programlisting>
    RAISE <replaceable class="parameter">level</replaceable>
	 <replaceable class="parameter">r">for</replaceable>le>'' [,
	  <replaceable class="parameter">identifier</replaceable> [...]];
    </programlisting>
    Inside the format, <quote>%</quote> is used as a placeholder for the
    subsequent comma-separated identifiers. Possible levels are
    DEBUG (silently suppressed in production running databases), NOTICE 
    (written into the database log and forwarded to the client application)
    and EXCEPTION (written into the database log and aborting the transaction).
</para>
</listitem>
</varlistentry>

<varlistentry>
<term>
Conditionals
</term>
<listitem>
<para>
    <programlisting>
    IF <replaceable>expression</replaceable> THEN
        <replaceable>statements</replaceable>
    [ELSE
        <replaceable>statements</replaceable>]
    END IF;
    </programlisting>
    The <replaceable>expression</replaceable> must return a value that
    at least can be casted into a boolean type.
</para>
</listitem>
</varlistentry>

<varlistentry>
<term>
Loops
</term>
<listitem>
<para>
    There are multiple types of loops.
    <programlisting>
    [&lt;&lt;label&gt;&gt;]
    LOOP
        <replaceable>statements</replaceable>
    END LOOP;
    </programlisting>
    An unconditional loop that must be terminated explicitly
    by an EXIT statement. The optional label can be used by
    EXIT statements of nested loops to specify which level of
    nesting should be terminated.
    <programlisting>
    [&lt;&lt;label&gt;&gt;]
    WHILE <replaceable>expression</replaceable> LOOP
        <replaceable>statements</replaceable>
    END LOOP;
    </programlisting>
    A conditional loop that is executed as long as the evaluation
    of <replaceable>expression</replaceable> is true.
    <programlisting>
    [&lt;&lt;label&gt;&gt;]
    FOR <replaceable>name</replaceable> IN [ REVERSE ]
<replaceable>le>express</replaceable>le> .. <replaceable>expression</replaceable> LOOP
        <replaceable>statements</replaceable>
    END LOOP;
    </programlisting>
    A loop that iterates over a range of integer values. The variable
    <replaceable>name</replaceable> is automatically created as type
    integer and exists only inside the loop. The two expressions giving
    the lower and upper bound of the range are evaluated only when entering
    the loop. The iteration step is always 1.
    <programlisting>
    [&lt;&lt;label&gt;&gt;]
    FOR <replaceable>record | row</replaceable> IN <replaceable>select_clause</replaceable> LOOP
        <replaceable>statements</replaceable>
    END LOOP;
    </programlisting>
    The record or row is assigned all the rows resulting from the select
    clause and the statements executed for each. If the loop is terminated
    with an EXIT statement, the last assigned row is still accessible 
    after the loop.
    <programlisting>
    EXIT [ <replaceable>label</replaceable> ] [ WHEN <replaceable>expression</replaceable> ];
    </programlisting>
    If no <replaceable>label</replaceable> given,
 the innermost loop is terminated and the
    statement following END LOOP is executed next.
 If <replaceable>label</replaceable> is given, it
    must be the label of the current or an upper level of nested loop
    blocks. Then the named loop or block is terminated and control
    continues with the statement after the loops/blocks corresponding
    END.
</para>
</listitem>
</varlistentry>

</variablelist>

</sect3>

<!-- **** PL/pgSQL trigger procedures **** -->

<sect3>
<title>Trigger Procedures</title>

<para>
    PL/pgSQL can be used to define trigger procedures. They are created
    with the usual CREATE FUNCTION command as a function with no
    arguments and a return type of OPAQUE.
</para>
<para>
    There are some <productname>Postgres</productname> specific details
    in functions used as trigger procedures.
</para>
<para>
    First they have some special variables created automatically in the 
    toplevel blocks declaration section. They are
</para>

<variablelist>

<varlistentry>
<term>
    NEW
</term>
<listitem>
<para>
    Datatype RECORD; variable holding the new database row on INSERT/UPDATE
    operations on ROW level triggers.
</para>
</listitem>
</varlistentry>

<varlistentry>
<term>
    OLD
</term>
<listitem>
<para>
    Datatype RECORD; variable holding the old database row on UPDATE/DELETE
    operations on ROW level triggers.
</para>
</listitem>
</varlistentry>

<varlistentry>
<term>
    TG_NAME
</term>
<listitem>
<para>
    Datatype name; variable that contains the name of the trigger actually
    fired.
</para>
</listitem>
</varlistentry>

<varlistentry>
<term>
    TG_WHEN
</term>
<listitem>
<para>
    Datatype text; a string of either 'BEFORE' or 'AFTER' depending on the
    triggers definition.
</para>
</listitem>
</varlistentry>

<varlistentry>
<term>
    TG_LEVEL
</term>
<listitem>
<para>
    Datatype text; a string of either 'ROW' or 'STATEMENT' depending on the
    triggers definition.
</para>
</listitem>
</varlistentry>

<varlistentry>
<term>
    TG_OP
</term>
<listitem>
<para>
    Datatype text; a string of 'INSERT', 'UPDATE' or 'DELETE' telling
    for which operation the trigger is actually fired.
</para>
</listitem>
</varlistentry>

<varlistentry>
<term>
    TG_RELID
</term>
<listitem>
<para>
    Datatype oid; the object ID of the table that caused the
    trigger invocation.
</para>
</listitem>
</varlistentry>

<varlistentry>
<term>
    TG_RELNAME
</term>
<listitem>
<para>
    Datatype name; the name of the table that caused the trigger
    invocation.
</para>
</listitem>
</varlistentry>

<varlistentry>
<term>
    TG_NARGS
</term>
<listitem>
<para>
    Datatype integer; the number of arguments given to the trigger
    procedure in the CREATE TRIGGER statement.
</para>
</listitem>
</varlistentry>

<varlistentry>
<term>
    TG_ARGV[]
</term>
<listitem>
<para>
    Datatype array of text; the arguments from the CREATE TRIGGER statement.
    The index counts from 0 and can be given as an expression. Invalid
    indices (&lt; 0 or &gt;= tg_nargs) result in a NULL value.
</para>
</listitem>
</varlistentry>

</variablelist>

<para>
    Second they must return either NULL or a record/row containing
    exactly the structure of the table the trigger was fired for.
    Triggers fired AFTER might always return a NULL value with no
    effect. Triggers fired BEFORE signal the trigger manager
    to skip the operation for this actual row when returning NULL.
    Otherwise, the returned record/row replaces the inserted/updated
    row in the operation. It is possible to replace single values directly
    in NEW and return that or to build a complete new record/row to
    return.
</para>
</sect3>

<!-- **** PL/pgSQL exceptions **** -->

<sect3>
<title>Exceptions</title>

<para>
    <productname>Postgres</productname> does not have a very smart
    exception handling model. Whenever the parser, planner/optimizer
    or executor decide that a statement cannot be processed any longer,
    the whole transaction gets aborted and the system jumps back
    into the mainloop to get the next query from the client application.
</para>
<para>
    It is possible to hook into the error mechanism to notice that this
    happens. But currently it's impossible to tell what really
    caused the abort (input/output conversion error, floating point
    error, parse error). And it is possible that the database backend
    is in an inconsistent state at this point so returning to the upper
    executor or issuing more commands might corrupt the whole database.
    And even if, at this point the information, that the transaction
    is aborted, is already sent to the client application, so resuming
    operation does not make any sense.
</para>
<para>
    Thus, the only thing PL/pgSQL currently does when it encounters
    an abort during execution of a function or trigger
    procedure is to write some additional DEBUG level log messages
    telling in which function and where (line number and type of
    statement) this happened.
</para>
</sect3>
</sect2>

<!-- **** PL/pgSQL Examples **** -->

<sect2>
<title>Examples</title>

<para>
Here are only a few functions to demonstrate how easy PL/pgSQL
functions can be written. For more complex examples the programmer
might look at the regression test for PL/pgSQL.
</para>

<para>
One painful detail of writing functions in PL/pgSQL is the handling
of single quotes. The functions source text on CREATE FUNCTION must
be a literal string. Single quotes inside of literal strings must be
either doubled or quoted with a backslash. We are still looking for
an elegant alternative. In the meantime, doubling the single qoutes
as in the examples below should be used. Any solution for this
in future versions of <productname>Postgres</productname> will be
upward compatible.
</para>

<sect3>
<title>Some Simple PL/pgSQL Functions</title>

<para>
    The following two PL/pgSQL functions are identical to their
    counterparts from the C language function discussion.

    <programlisting>
    CREATE FUNCTION add_one (int4) RETURNS int4 AS '
        BEGIN
            RETURN $1 + 1;
        END;
    ' LANGUAGE 'plpgsql';
    </programlisting>

    <programlisting>
    CREATE FUNCTION concat_text (text, text) RETURNS text AS '
        BEGIN
            RETURN $1 || $2;
        END;
    ' LANGUAGE 'plpgsql';
    </programlisting>
</para>

</sect3>

<sect3>
<title>PL/pgSQL Function on Composite Type</title>

<para>
    Again it is the PL/pgSQL equivalent to the example from
    The C functions.

    <programlisting>
    CREATE FUNCTION c_overpaid (EMP, int4) RETURNS bool AS '
        DECLARE
            emprec ALIAS FOR $1;
            sallim ALIAS FOR $2;
        BEGIN
            IF emprec.salary ISNULL THEN
                RETURN ''f'';
            END IF;
            RETURN emprec.salary > sallim;
        END;
    ' LANGUAGE 'plpgsql';
    </programlisting>
</para>

</sect3>

<sect3>
<title>PL/pgSQL Trigger Procedure</title>

<para>
    This trigger ensures, that any time a row is inserted or updated
    in the table, the current username and time are stamped into the
    row. And it ensures that an employees name is given and that the
    salary is a positive value.

    <programlisting>
    CREATE TABLE emp (
        empname text,
        salary int4,
        last_date datetime,
        last_user name);

    CREATE FUNCTION emp_stamp () RETURNS OPAQUE AS
        BEGIN
            -- Check that empname and salary are given
            IF NEW.empname ISNULL THEN
                RAISE EXCEPTION ''empname cannot be NULL value'';
            END IF;
            IF NEW.salary ISNULL THEN
                RAISE EXCEPTION ''% cannot have NULL salary'', NEW.empname;
            END IF;

            -- Who works for us when she must pay for?
            IF NEW.salary < 0 THEN
                RAISE EXCEPTION ''% cannot have a negative salary'', NEW.empname;
            END IF;

            -- Remember who changed the payroll when
            NEW.last_date := ''now'';
            NEW.last_user := getpgusername();
            RETURN NEW;
        END;
    ' LANGUAGE 'plpgsql';

    CREATE TRIGGER emp_stamp BEFORE INSERT OR UPDATE ON emp
        FOR EACH ROW EXECUTE PROCEDURE emp_stamp();
    </programlisting>
</para>

</sect3>

</sect2>

</sect1>

<!-- **********
     * The procedural language PL/Tcl
     **********
-->

<sect1>
<title>PL/Tcl</title>

<para>
    PL/Tcl is a loadable procedural language for the
    <productname>Postgres</productname> database system
    that enables the Tcl language to be used to create functions and
    trigger-procedures.
</para>

<para>
    This package was originally written by Jan Wieck.
</para>

<!-- **** PL/Tcl overview **** -->

<sect2>
<title>Overview</title>

<para>
    PL/Tcl offers most of the capabilities a function
    writer has in the C language, except for some restrictions.
</para>
<para>
    The good restriction is, that everything is executed in a safe
    Tcl-interpreter. In addition to the limited command set of safe Tcl, only
    a few commands are available to access the database over SPI and to raise
    messages via elog(). There is no way to access internals of the
    database backend or gaining OS-level access under the permissions of the
    <productname>Postgres</productname> user ID like in C.
    Thus, any unprivileged database user may be
    permitted to use this language.
</para>
<para>
    The other, internal given, restriction is, that Tcl procedures cannot
    be used to create input-/output-functions for new data types.
</para>
<para>
    The shared object for the PL/Tcl call handler is automatically built
    and installed in the <productname>Postgres</productname>
    library directory if the Tcl/Tk support is specified
    in the configuration step of the installation procedure.
</para>
</sect2>

<!-- **** PL/Tcl description **** -->

<sect2>
<title>Description</title>

<sect3>
<title><productname>Postgres</productname> Functions and Tcl Procedure Names</title>

<para>
    In <productname>Postgres</productname>, one and the 
    same function name can be used for
    different functions as long as the number of arguments or their types
    differ. This would collide with Tcl procedure names. To offer the same
    flexibility in PL/Tcl, the internal Tcl procedure names contain the object
    ID of the procedures pg_proc row as part of their name. Thus, different
    argtype versions of the same <productname>Postgres</productname> 
    function are different for Tcl too.
</para>

</sect3>

<sect3>
<title>Defining Functions in PL/Tcl</title>

<para>
    To create a function in the PL/Tcl language, use the known syntax

    <programlisting>
    CREATE FUNCTION <replaceable>funcname</replaceable>
 <replaceable>ceable>argumen</replaceable>ceable>) RETURNS
       <replaceable>returntype</replaceable> AS '
        # PL/Tcl function body
    ' LANGUAGE 'pltcl';
    </programlisting>

    When calling this function in a query, the arguments are given as
    variables $1 ... $n to the Tcl procedure body. So a little max function
    returning the higher of two int4 values would be created as:

    <programlisting>
    CREATE FUNCTION tcl_max (int4, int4) RETURNS int4 AS '
        if {$1 > $2} {return $1}
	return $2
    ' LANGUAGE 'pltcl';
    </programlisting>

    Composite type arguments are given to the procedure as Tcl arrays.
    The element names
    in the array are the attribute names of the composite
    type. If an attribute in the actual row
    has the NULL value, it will not appear in the array! Here is
    an example that defines the overpaid_2 function (as found in the
    older <productname>Postgres</productname> documentation) in PL/Tcl

    <programlisting>
    CREATE FUNCTION overpaid_2 (EMP) RETURNS bool AS '
        if {200000.0 < $1(salary)} {
            return "t"
        }
        if {$1(age) < 30 && 100000.0 < $1(salary)} {
            return "t"
        }
        return "f"
    ' LANGUAGE 'pltcl';
    </programlisting>
</para>

</sect3>

<sect3>
<title>Global Data in PL/Tcl</title>

<para>
    Sometimes (especially when using the SPI functions described later) it
    is useful to have some global status data that is held between two
    calls to a procedure. 
    All PL/Tcl procedures executed in one backend share the same
    safe Tcl interpreter.
    To help protecting PL/Tcl procedures from side effects,
    an array is made available to each procedure via the upvar
    command. The global name of this variable is the procedures internal
    name and the local name is GD.
</para>
</sect3>

<sect3>
<title>Trigger Procedures in PL/Tcl</title>

<para>
    Trigger procedures are defined in <productname>Postgres</productname>
    as functions without
    arguments and a return type of opaque. And so are they in the PL/Tcl
    language.
</para>
<para>
    The informations from the trigger manager are given to the procedure body
    in the following variables:
</para>
<variablelist>

<varlistentry>
<term><replaceable class="Parameter">
$TG_name
</replaceable></term>
<listitem>
<para>
    The name of the trigger from the CREATE TRIGGER statement.
</para>
</listitem>
</varlistentry>

<varlistentry>
<term><replaceable class="Parameter">
$TG_relid
</replaceable></term>
<listitem>
<para>
    The object ID of the table that caused the trigger procedure
    to be invoked.
</para>
</listitem>
</varlistentry>

<varlistentry>
<term><replaceable class="Parameter">
$TG_relatts
</replaceable></term>
<listitem>
<para>
    A Tcl list of the tables field names prefixed with an empty list element.
    So looking up an element name in the list with the lsearch Tcl command
    returns the same positive number starting from 1 as the fields are numbered
    in the pg_attribute system catalog.
</para>
</listitem>
</varlistentry>

<varlistentry>
<term><replaceable class="Parameter">
$TG_when
</replaceable></term>
<listitem>
<para>
    The string BEFORE or AFTER depending on the event of the trigger call.
</para>
</listitem>
</varlistentry>

<varlistentry>
<term><replaceable class="Parameter">
$TG_level
</replaceable></term>
<listitem>
<para>
    The string ROW or STATEMENT depending on the event of the trigger call.
</para>
</listitem>
</varlistentry>

<varlistentry>
<term><replaceable class="Parameter">
$TG_op
</replaceable></term>
<listitem>
<para>
    The string INSERT, UPDATE or DELETE depending on the event of the 
    trigger call.
</para>
</listitem>
</varlistentry>

<varlistentry>
<term><replaceable class="Parameter">
$NEW
</replaceable></term>
<listitem>
<para>
    An array containing the values of the new table row on INSERT/UPDATE
    actions, or empty on DELETE.
</para>
</listitem>
</varlistentry>

<varlistentry>
<term><replaceable class="Parameter">
$OLD
</replaceable></term>
<listitem>
<para>
    An array containing the values of the old table row on UPDATE/DELETE
    actions, or empty on INSERT.
</para>
</listitem>
</varlistentry>

<varlistentry>
<term><replaceable class="Parameter">
$GD
</replaceable></term>
<listitem>
<para>
    The global status data array as described above.
</para>
</listitem>
</varlistentry>

<varlistentry>
<term><replaceable class="Parameter">
$args
</replaceable></term>
<listitem>
<para>
    A Tcl list of the arguments to the procedure as given in the
    CREATE TRIGGER statement. The arguments are also accessible as $1 ... $n
    in the procedure body.
</para>
</listitem>
</varlistentry>

</variablelist>

<para>
    The return value from a trigger procedure is one of the strings OK or SKIP,
    or a list as returned by the 'array get' Tcl command. If the return value
    is OK, the normal operation (INSERT/UPDATE/DELETE) that fired this trigger
    will take place. Obviously, SKIP tells the trigger manager to silently
    suppress the operation. The list from 'array get' tells PL/Tcl
    to return a modified row to the trigger manager that will be inserted instead
    of the one given in $NEW (INSERT/UPDATE only). Needless to say that all
    this is only meaningful when the trigger is BEFORE and FOR EACH ROW.
</para>
<para>
    Here's a little example trigger procedure that forces an integer value
    in a table to keep track of the # of updates that are performed on the
    row. For new row's inserted, the value is initialized to 0 and then
    incremented on every update operation:

    <programlisting>
    CREATE FUNCTION trigfunc_modcount() RETURNS OPAQUE AS '
        switch $TG_op {
            INSERT {
                set NEW($1) 0
            }
            UPDATE {
                set NEW($1) $OLD($1)
                incr NEW($1)
            }
            default {
                return OK
            }
        }
        return [array get NEW]
    ' LANGUAGE 'pltcl';

    CREATE TABLE mytab (num int4, modcnt int4, desc text);

    CREATE TRIGGER trig_mytab_modcount BEFORE INSERT OR UPDATE ON mytab
        FOR EACH ROW EXECUTE PROCEDURE trigfunc_modcount('modcnt');
    </programlisting>

</para>
</sect3>

<sect3>
<title>Database Access from PL/Tcl</title>

<para>
    The following commands are available to access the database from
    the body of a PL/Tcl procedure:
</para>

<variablelist>

<varlistentry>
<term>
elog <replaceable>level</replaceable> <replaceable>msg</replaceable>
</term>
<listitem>
<para>
    Fire a log message. Possible levels are NOTICE, WARN, ERROR,
    FATAL, DEBUG and NOIND
    like for the elog() C function.
</para>
</listitem>
</varlistentry>

<varlistentry>
<term>
quote <replaceable>string</replaceable>
</term>
<listitem>
<para>
    Duplicates all occurences of single quote and backslash characters.
    It should be used when variables are used in the query string given
    to spi_exec or spi_prepare (not for the value list on spi_execp).
    Think about a query string like

    <programlisting>
    "SELECT '$val' AS ret"
    </programlisting>

    where the Tcl variable val actually contains "doesn't". This would result
    in the final query string

    <programlisting>
    "SELECT 'doesn't' AS ret"
    </programlisting>

    what would cause a parse error during spi_exec or spi_prepare.
    It should contain

    <programlisting>
    "SELECT 'doesn''t' AS ret"
    </programlisting>

    and has to be written as

    <programlisting>
    "SELECT '[ quote $val ]' AS ret"
    </programlisting>
</para>
</listitem>
</varlistentry>

<varlistentry>
<term>
spi_exec ?-count <replaceable>n</replaceable>? ?-array
<replaceable>>nam</replaceable>>?<replaceable>e>que</replaceable>e> ?<replaceable>loop-body</replaceable>?
</term>
<listitem>
<para>
    Call parser/planner/optimizer/executor for query.
    The optional -count value tells spi_exec the maximum number of rows
    to be processed by the query.
</para>
<para>
    If the query is
    a SELECT statement and the optional loop-body (a body of Tcl commands
    like in a foreach statement) is given, it is evaluated for each
    row selected and behaves like expected on continue/break. The values
    of selected fields are put into variables named as the column names. So a

    <programlisting>
    spi_exec "SELECT count(*) AS cnt FROM pg_proc"
    </programlisting>

    will set the variable $cnt to the number of rows in the pg_proc system
    catalog. If the option -array is given, the column values are stored
    in the associative array named 'name' indexed by the column name
    instead of individual variables.

    <programlisting>
    spi_exec -array C "SELECT * FROM pg_class" {
        elog DEBUG "have table $C(relname)"
    }
    </programlisting>

    will print a DEBUG log message for every row of pg_class. The return value
    of spi_exec is the number of rows affected by query as found in
    the global variable SPI_processed.
</para>
</listitem>
</varlistentry>

<varlistentry>
<term>
spi_prepare <replaceable>query</replaceable> <replaceable>typelist</replaceable>
</term>
<listitem>
<para>
    Prepares AND SAVES a query plan for later execution. It is a bit different
    from the C level SPI_prepare in that the plan is automatically copied to the
    toplevel memory context. Thus, there is currently no way of preparing a
    plan without saving it.
</para>
<para>
    If the query references arguments, the type names must be given as a Tcl
    list. The return value from spi_prepare is a query ID to be used in
    subsequent calls to spi_execp. See spi_execp for a sample.
</para>
</listitem>
</varlistentry>

<varlistentry>
<term>
spi_exec ?-count <replaceable>n</replaceable>? ?-array
<replaceable>>nam</replaceable>>? ?-nulls<replaceable>e>s</replaceable>e><replaceable>le>qu</replaceable>le<replaceable>ble>value</replaceable>ble>? ?<replaceable>loop-body</replaceable>?
</term>
<listitem>
<para>
    Execute a prepared plan from spi_prepare with variable substitution.
    The optional -count value tells spi_execp the maximum number of rows
    to be processed by the query.
</para>
<para>
    The optional value for -nulls is a string of spaces and 'n' characters
    telling spi_execp which of the values are NULL's. If given, it must
    have exactly the length of the number of values.
</para>
<para>
    The queryid is the ID returned by the spi_prepare call.
</para>
<para>
    If there was a typelist given to spi_prepare, a Tcl list of values of
    exactly the same length must be given to spi_execp after the query. If
    the type list on spi_prepare was empty, this argument must be omitted.
</para>
<para>
    If the query is a SELECT statement, the same as described for spi_exec
    happens for the loop-body and the variables for the fields selected.
</para>
<para>
    Here's an example for a PL/Tcl function using a prepared plan:

    <programlisting>
    CREATE FUNCTION t1_count(int4, int4) RETURNS int4 AS '
        if {![ info exists GD(plan) ]} {
            # prepare the saved plan on the first call
            set GD(plan) [ spi_prepare \\
                    "SELECT count(*) AS cnt FROM t1 WHERE num &gt;= \\$1 AND num &lt;= \\$2" \\
                    int4 ]
        }
        spi_execp -count 1 $GD(plan) [ list $1 $2 ]
        return $cnt
    ' LANGUAGE 'pltcl';
    </programlisting>

    Note that each backslash that Tcl should see must be doubled in
    the query creating the function, since the main parser processes
    backslashes too on CREATE FUNCTION.
    Inside the query string given to spi_prepare should
    really be dollar signs to mark the parameter positions and to not let
    $1 be substituted by the value given in the first function call.
</para>
</listitem>
</varlistentry>

<varlistentry>
<term>
Modules and the unknown command
</term>
<listitem>
<para>
    PL/Tcl has a special support for things often used. It recognizes two
    magic tables, pltcl_modules and pltcl_modfuncs.
    If these exist, the module 'unknown' is loaded into the interpreter
    right after creation. Whenever an unknown Tcl procedure is called,
    the unknown proc is asked to check if the procedure is defined in one
    of the modules. If this is true, the module is loaded on demand.
    To enable this behavior, the PL/Tcl call handler must be compiled
    with -DPLTCL_UNKNOWN_SUPPORT set.
</para>
<para>
    There are support scripts to maintain these tables in the modules
    subdirectory of the PL/Tcl source including the source for the
    unknown module that must get installed initially.
</para>
</listitem>
</varlistentry>

</variablelist>

</sect3>

</sect2>
</sect1>
</chapter>

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